The present invention relates to protective coatings for metal alloy components, such as the working components of gas turbine engines that are exposed to high temperature gas environments and severe operating conditions. More particularly, the invention relates to a new type of protective bond coating that includes a bi-layer bond coat applied to a metal substrate using high velocity oxy-fuel (“HVOF”) thermal spraying. In one embodiment, bond coatings in accordance with the invention can be used in combination with a thermal barrier coating (“TBC”). However, the invention can also take other forms, such as a stand alone overlay coating.
Exemplary bi-layer bond coatings in accordance with the invention include a dense first inner layer that provides oxidation protection to the metal substrate, and a second outer layer having controlled porosity that tends to promote roughness and aids adherence of the TBC to the bond coat. Preferably, the second less dense layer of the bi-layer bond coat is formed from a mixture of metallic powder and polyester to control and adjust the porosity to the desired level. Together, the layers enhance adherence between the bond coat and TBC and thereby improve the overall life of the coating system.
The present invention also relates to a new method for forming a bi-layer bond coating in which both layers of the coating are applied to the metal alloy components using HVOF thermal spraying. As discussed below, HVOF has not been used in the past for coatings containing polyester to control coating porosity, particularly as part of a multiple layer bond coating applied to metal substrates.
The invention is particularly well suited for use with gas turbine engine components since it is known that the operating conditions for such components can be thermally and chemically severe. By necessity, the surfaces of the metal substrates used to form the turbine, combustor and augmentor components must exhibit greater than average mechanical strength, durability, oxidation resistance, and erosion resistance in hostile, high temperature gas environments.
In recent years, significant advances have been made in gas turbine systems by incorporating high temperature alloys of iron, nickel and cobalt-based compositions in the base coatings applied to the metal substrate of the turbine components, particularly buckets and nozzles. For example, some prior coating systems have included a top layer comprising a thermal-insulating ceramic (typically referred to as a thermal barrier coating or “TBC”), together with an environmentally-resistant lower bond coat adhered to the alloy metal substrate. Metal oxides, such as zirconia (ZrO2) that are partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or another oxide, have been used to form the thermal-insulating ceramic layer using air plasma spray (“APS”), vacuum plasma spray (“VPS”) or physical vapor deposition techniques such as electron beam physical vapor deposition (“EBPVD”). See commonly-assigned U.S. Pat. No. 5,981,088 (disclosing the use of yttria-stabilized zirconia deposited by EBPVD in order to improve thermal cycle fatigue properties). See also, U.S. Pat. No. 5,817,372 (describing a process for depositing a bond coat for a thermal barrier coating system); U.S. Pat. No. 6,165,628 (disclosing protective coatings for metal-based substrates and related processes); U.S. Pat. No. 6,274,201 describing protective coatings for metal-based substrates and related processes); and U.S. Pat. No. 6,368,672 (disclosing a method for forming a thermal barrier coating system for turbine engine components).
The objectives of an effective bond coat on metal substrates are two-fold. First, the coating must form a dense, protective and adherent layer that guards the underlying base material against oxidation, corrosion, and degradation. Second, the coating should serve to promote adherence of the ceramic layer. For thermal spray coatings, a high degree of surface roughness is required to provide mechanical interlocking; EB-PVD TBCs require much smoother interface roughness to allow more uniform growth of the TBC columns. Bond layers are typically made of alloys such as MCrAlY, where M represents a metal such as Ni, Co, or Fe. Aluminide bond coats are often used for EB-PVD TBC. As bond coat compositions have become more complex, it has become increasingly difficult to obtain both the higher required strength levels (particularly at maximum gas turbine operating temperatures) and a satisfactory level of corrosion and oxidation resistance. The trend in recent years towards higher gas turbine firing temperatures has made the oxidation, corrosion and degradation problems even more difficult.
Typically, bond coats used in prior systems have been based on oxidation-resistant alloys such as MCrAlY or a diffusion aluminide or platinum aluminide that forms an oxidation-resistant intermetallic. The dense coatings formed from such compositions protect the underlying alloy by forming an oxidation barrier for the substrate at the outermost surface of the bond coat. This oxidation barrier is typically a dense adherent aluminum oxide layer (sometimes called an “alumina scale”) that forms at the elevated temperatures. The oxide scale tends to protect the bond coat from continued oxidation. Plasma-sprayed ceramic layers, for example, adhere well to the bond coat if the bond has a relatively rough surface. The rough surface of the bond coat enhances mechanical adherence though interlocking of the bond coat and TBC microstructures.
A description of this known relationship between thermal insulating ceramic layers and metal substrate bond coats appears in commonly-owned U.S. Pat. No. 5,817,372 to Zheng, the disclosure of which is hereby incorporated by reference. The '372 patent notes that the strength and integrity of the bond between a thermal insulating ceramic layer and bond coat often depends on the deposition technique involved. That is, the structure and physical properties of bond coats are dependent on the process and equipment by which they are deposited.
In the past, bond coats have been applied by thermal spraying, e.g., APS, VPS and HVOF techniques, all of which entail deposition of the bond coat using a metal powder. While bond coats deposited by such techniques have been employed successfully, each has distinct advantages and disadvantages, depending on the desired application. For example, with VPS, very little oxidation of the metal particle occurs during deposition, and thus the resulting bond coat tends to be dense, relatively free of oxides, and exhibits a high temperature capability due to the inherent ability to grow a continuous protective oxide scale. VPS processes have a relatively low heat capacity to melt the spray powder and thus typically employ powders having very fine particle size distribution. As a result, VPS bond coats tend to be dense, but have relatively smooth surfaces (typically 200 to 350 microinches, i.e., about 4 to about 9 μm)). Consequently, many plasma-sprayed ceramic layers do not adhere well to underlying VPS bond coats. U.S. Pat. No. 5,817,372 describes a VPS coating with improved roughness generated by use of coarse powders that do not completely melt during the deposition process.
Air plasma spray (APS) has a high heat capacity that enables the melting of relatively large particles and permits the use of metal powders that yield bond coats having a somewhat rougher surface than those formed by VPS. Thus, the adhesion of a ceramic layer to an APS bond coat is normally enhanced by the rough APS bond coat surface. The particle size distribution of such powders is also broad, thereby allowing the finer particles to partially fill the interstices between larger particles and reduce porosity. However, the finer particles used in APS are prone to oxidation during the spraying process, typically resulting in a bond coat having a very high oxide content. The entrapped oxides and the larger particle size utilized in APS coatings tend to promote porosity in the coating. Consequently, APS bond coats inherently contain relatively high levels of oxides and are less dense than VPS bond coats, making them more prone to oxidation than VPS coats.
The prior art methods used to apply bond coats fall into three general categories: (1) fully dense coats produced by APS, VPS, or HVOF processes; (2) fully porous coats formed by APS processing; or (3) a bi-layers of dense coating followed by one or more porous layers. The bi-layer coatings are oftentimes produced by two different processes (typically HVOF or VPS for the dense layer and APS for the porous layer). The bi-layer structure can also be formed by modification of particle size and/or spray parameters to create a rougher or more porous outer layer. As noted above, heretofore HVOF has not been used to form both the dense and porous layers.
The use of two different thermal spray processes to form the bi-layer bond coating creates an undesirable complication in the manufacturing process. Typically, the known HVOF/APS bi-layer coatings require an intermediate vacuum heat treatment after the HVOF layer is applied. The '732 patent identified above discusses one such bi-layer bond coating system consisting of two HVOF layers utilizing different powder sizes to provide increased roughness at the surface. However, the outer layer is close to maximum density and lacks the mechanical advantages of a more porous coating layer. It is now recognized that the use of a single HVOF system to apply both the dense and porous layers of the bi-layer coating could greatly simplify the manufacturing process and lead to cost reductions due to reduced cycle time. A reduction in cycle time could also be achieved by eliminating the intermediate vacuum heat treatment of the HVOF described in the '372 patent.
Although some prior art air plasma sprayed coatings have used polyester powder as a fugitive filler in order to adjust and control porosity (for example, to produce abradable surface coatings), the use of polyester filler to create porosity in bi-layer HVOF bond coatings is not known. Bond coats deposited by HVOF techniques are known to be very sensitive to the particle size distribution of the powder because of the relatively low spray temperature of the HVOF. Thus, the thermal spray parameters normally must be adjusted for powders having a very narrow range of particle size distribution. Further, in order to produce an effective bond coat using an HVOF process, a coarse powder must be used in order to achieve adequate surface roughness. However, because coarse particles cannot typically be fully melted at the lower HVOF temperature parameters, HVOF bond coats made from coarse powders typically exhibit higher porosity and poorer bonding between particles. HVOF coatings made from fine powders are more dense but lack adequate roughness for good TBC adherence.
Thus, despite recent developments in bond coatings, including some bi-layer coatings used in combination with TBC, there remains a need in the art for improved protective coatings on metal alloy components exposed to high temperature environments in gas turbine engine components. The need also exists for improved methods of applying such coatings to key turbine components exposed to hostile conditions at high temperatures.